1. Field of the Invention
The present invention relates in general to rotation speed control systems of an internal combustion engine, and more particularly to an idling speed control system of an internal combustion engine, which can precisely control the idling speed of the engine to a desired level.
2. Description of the Prior art
In known idling speed control systems of an automotive internal combustion engine, the amount of intake air is feedback-controlled in accordance with a difference between an actual rotation speed of the engine detected by a crankangle sensor and a target rotation speed, and usually a so-called "PI control" is employed for gently increasing or decreasing the intake air amount. The torque produced by the engine is generally proportional to the amount of air-fuel mixture fed to the engine, that is, to the intake air amount. However, since the change in rotation speed of the engine is given in the form of the integral of the torque change, the rotation speed change is somewhat delayed as compared with the change of the intake air amount. Thus, in order to avoid excessive delay, usually the change in intake air amount is controlled relatively gently with respect to the rotation speed change.
Japanese Patent First Provisional Publications Nos. 1-211640 and 2-78748 show measures for improving the above-mentioned slow control. That is, in the measure of the former publication, a so-called "feed-forward" control is employed in which any disturbance causing the fluctuation of rotation speed is detected and a corresponding amount of intake air is instantly fed to the engine based on the detected disturbance. In the measure of the latter publication, a control is employed in which a lowering rate of the rotation speed is monitored and when a sudden lowering of the rotation speed is detected, an intake air compensating degree is increased.
However, in hitherto proposed conventional idling speed control systems including those of the above-mentioned publications, satisfied performance has not be obtained due to their inherent constructions. That is, in the type wherein the feedback control is applied to the intake air amount based on the rotation speed change, a marked control delay occurs inevitably. Thus, in this type, high responsive control is not obtained. While, in the measure of the 1-211640 publication, it is almost impossible to set corrected intake air amount to every types of disturbances. Besides, the intake air compensation can not be properly applied to a rotation speed change which is caused by non-predictable disturbance, such as, change in combustion condition of the engine or the like. Furthermore, in the measure of the 2-78748 publication, it is almost impossible to provide the air intake with a precisely controlled compensation. In fact, suppression of an engine stall tends to induce an excessively high idling speed of the engine.